Objective lens, optical pickup device, and optical recording/reproducing apparatus

ABSTRACT

An objective lens for converging light emitted from a light source on an optical recording medium to record and reproduce information consists of a single lens having at least one aspheric surface. The following conditional expressions (1) to (3) are satisfied:
 
 N ≦1.75  (1)
 
0.5&lt; f/f   1 &lt;0.6  (2)
 
0.8&lt; d /( NA·De )&lt;1.0  (3)
 
where
         N denotes a refractive index,   f denotes a focal length (mm),   f 1  denotes a focal length (mm) of a light source side surface,   d denotes a thickness (mm) on an optical axis,   NA denotes a numerical aperture on an optical recording medium side, and   De denotes an effective aperture (mm) of the light source side surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe Japanese Patent Application No. 2008-129205 filed on May 16, 2008;the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to an objective lens, an optical pickup device,and an optical recording/reproducing apparatus. Specifically, theinvention relates to an objective lens for use in an optical pickupapparatus for converging light emitted from a light source onto anoptical recording medium to perform at least one of operations ofrecording and reproducing information, an optical pickup device havingthe objective lens, and an optical recording/reproducing apparatusequipped with the optical pickup device.

2. Description of the Related Art

Generally, various optical recording media such as DVD (digitalversatile disk) and CD (compact disk) have been used in order to recordaudio information, video information, or data information for computer.As an amount of information to be handled rapidly increases, an increasein storage capacity density of an optical recording medium has beenstrongly demanded. It has been known that a decrease in wavelength ofused light and an increase in numerical aperture (hereinafter, it isreferred to as an NA) of an objective lens for an optical pickup deviceare effective to increase the storage capacity density of an opticalrecording medium. Recently, for a BD (blu-ray disk), a semiconductorlaser having an output wavelength of about 405 nm is used as a lightsource and an objective lens having an NA of 0.7 or more is used. Such ablu-ray disk (hereinafter, it is referred to as BD) having a capacity of25 GB per one layer of one side has been widely used in practice. In thespecifications of the BD, an NA and a thickness of a protection layer ofan optical recording medium are set totally different from values of DVDand CD. In the current specification, an NA is 0.85, and a thickness ofa protection layer is 0.1 mm.

However, in future, as might be expected, an increase in density will bedemanded more and more, but it may be hard to satisfy this demand bypromoting a decrease in wavelength. This is because opticaltransmittance of lens materials is rapidly reduced in the range of awavelength less than 350 nm and thus, it is hard to obtain sufficientoptical efficiency in practice. For this reason, the other way forachieving high storage capacity density is to further increase an NA ofthe objective lens.

When an objective lens with a high numerical aperture (hereinafter, itis referred to as a high NA) for an optical pickup device is designed, asingle lens structure is effective to solve problems such as an increasein process number at the time of assembly, deterioration in productionefficiency, and an increase in cost. For example, known objective lensesfor an optical pickup device having a relatively high NA single lens aredescribed in JP 2001-324673 A (corresponding to U.S. Pat. Nos. 6,411,442and 6,512,640) and Japanese patent No. 3712628 (corresponding to U.S.Pat. No. 6,744,568).

JP 2001-324673 A describes an objective lens that can achieve excellentimage height characteristics by setting a ratio of a thickness d of thelens on the optical axis to a focal length f thereof in a predeterminedrange. Japanese patent No. 3712628 describes an aspheric objective lenshaving a light source side surface that is formed as a convex surfaceand an optical recording medium side surface that is formed in a shapesubstantially flatter than the light source side surface.

Meanwhile, in the high-NA objective lens for an optical pickup device,it is difficult to secure a working distance (WD) that can preventcollision between the lens and an optical recording medium whilemaintaining compactness of the lens system. In this point of view, theobjective lens described in JP 2001-324673 A has a room for improvement.Also, in focusing and tracking controls, it is preferable to drive alens at higher speed. For this reason, it is also required to reduce aweight of a lens.

However, in the objective lens described in Japanese patent No. 3712628,a refractive index of the lens component material is set high, and thusa weight of lens becomes large. In lens material, particularly in glassmaterial, when a refractive index thereof is high, a specific gravitygenerally tends to increase. For example, a specific gravity of amaterial having a refractive index of about 1.60 is about 2.8, but aspecific gravity of a material having a refractive index of about 1.85is about 4.3. Accordingly, assuming that volumes of lenses are the same,a weight of a lens formed of a material having a refractive index ofabout 1.85 is 1.5 times as large as a weight of a lens formed of amaterial having a refractive index of about 1.60. This causes a problemin that it is difficult to drive a lens at a high speed in focusing andtracking controls since the weight of the lens described in Japanesepatent No. 3712628 becomes large in any cases.

SUMMARY OF THE INVENTION

The invention has been made in view of the above circumstances andprovides an objective lens capable of achieving reduction in weight witha sufficient working distance while maintaining high optical performanceas an objective lens for an optical pickup device. The invention alsoprovides an optical pickup device having the objective lens and anoptical recording/reproducing apparatus equipped with the optical pickupdevice.

[1] According to an aspect of the invention, an objective lens forconverging light emitted from a light source onto an optical recordingmedium to record and reproduce information consists of a single lenshaving at least one aspheric surface. The following conditionalexpressions (1) to (3) are satisfied:N≦1.75  (1)0.5<f/f1<0.6  (2)0.8<d/(NA·De)<1.0  (3)where

N denotes a refractive index of the single lens,

f denotes a focal length, in mm, of the single lens,

f1 denotes a focal length, in mm, of a light source side surface of thesingle lens,

d denotes a thickness, in mm, of the single lens on an optical axis,

NA denotes a numerical aperture of the single lens on an opticalrecording medium side, and

De denotes an effective aperture, in mm, of the light source sidesurface of the single lens.

[2] According to another aspect of the invention, an objective lens forconverging light emitted from a light source onto an optical recordingmedium to record and reproduce information consists of a single lenshaving at least one aspheric surface. The following conditionalexpressions (1), (2), and (4) are satisfied:N≦1.75  (1)0.5<f/f1<0.6  (2)0.3<d/Do<0.8  (4)where

N denotes a refractive index of the single lens,

f denotes a focal length, in mm, of the single lens,

f1 denotes a focal length, in mm, of a light source side surface of thesingle lens,

d denotes a thickness, in mm, of the single lens on an optical axis, and

Do denotes an outer diameter, in mm, of the single lens.

f1 in the expression (2) is defined by the following expression (A):f ₁ =N×R1/(N−1)  (A)where N denotes the refractive index of the objective lens, and

R1 denotes a curvature of radium (mm) of the light source side surfaceof the objective lens near the optical axis.

Here, in this specification, the “near the optical axis” means a minuterange including the optical axis and is also defined as a “paraxialrange” in which approximation of sin φ=φ can be established when theangle of φ is defined between the optical axis and a straight line thatconnects the center of curvature of a lens with a point on a lenssurface on which a ray is incident.

[3] Also, in the objective lens of any one of [1] to [2], the objectivelens may be formed in a biconvex shape near the optical axis.

[4] Also, in the objective lens of any one of [1] to [3], an opticalrecording medium side surface of the objective lens may be formed in aconvex shape near the optical axis, and be formed in a concave shape ata predetermined position distant from the optical axis.[5] Also, in the objective lens of any one of [1] to [4], a mass of theobjective lens may be equal to or less than 0.5 grams.

Here, the mass of objective lens means a mass of the whole objectivelens. For example, when the objective lens has a flange portion thatdoes not function to converge incident rays, it is defined as a massincluding a mass of the flange portion.

[6] Also, in the objective lens of any one of [1] to [5], the followingconditional expression (5) may be satisfied:0.50<g/d<0.80  (5)where g denotes a distance, in mm, from a tangential plane that isperpendicular to the optical axis and is tangent to a vertex of thelight source side surface of the single lens to a center of gravity ofthe single lens.[7] Also, in the objective lens according of any one of [1] to [6], anumerical aperture of the single lens on the optical recording mediumside may be not less than 0.70 and not more than 0.98.[8] In the objective lens of any one of [1] to [7], a wavelength of thelight may be not less than 400.0 nm and not more than 410.0 nm.[9] In the objective lens of [8], the numerical aperture of the singlelens on the optical recording medium side may be not less than 0.85, anda thickness of a protection layer of the optical recording medium may benot less than 0.075 mm and not more than 0.1 mm.

The sentence “the numerical aperture of the single lens on the opticalrecording medium side is not less than 0.85, and a thickness of theprotection layer of the optical recording medium is not less than 0.075mm and not more than 0.1 mm” means that the objective lenses of [1] and[2] can converge used light almost without aberration (for example,wavefront aberration is not more than 0.07 λRMS) while satisfying theabove conditions.

[10] In the objective lens of [8], the numerical aperture of the singlelens on the optical recording medium side may be not less than 0.85, anRMS (Root Means Square) of wavefront aberration may be set to beminimized at a position distant t1 mm from a surface of the opticalrecording medium toward the inside of the optical recording medium, andt1 may be not less than 0.075 mm and not more than 0.1 mm.

Here, N, f, f₁, NA, De, and wavefront aberration are values at thewavelength of used light.

Here, Do is defined as a diameter of an arc constituting an outer shapeof the lens in consideration of the outer shape of the lens including aflange portion in a plane perpendicular to the optical axis.

Furthermore, the numerical aperture of the objective lens can bedetermined based on the focal length and the effective diameter or theaperture diameter that is determined by the aperture diaphragm disposedtogether with the objective lens, when the effective diameter or theaperture diameter is already known. However, when these value areunknown, the maximum lens diameter that can converge the used lightalmost without aberration (for example, wavefront aberration is not morethan 0.07 λRMS) or the maximum lens diameter that can converge the usedlight to a spot diameter required to record or reproduce information isset as a practical effective diameter of the objective lens, and thenumerical aperture can be determined based on this practical effectivediameter.

[11] According to further another aspect of the invention, an opticalpickup device includes the objective lens of any one of [1] to [10].

[12] According to still another aspect of the invention, an opticalrecording/reproducing apparatus includes the optical pickup device of[11].

Furthermore, the optical recording/reproducing apparatus may have only afunction of recording or reproducing, and may have both of the functionsof recording and reproducing. Further, the optical recording/reproducingapparatus may record onto one optical recording medium and reproducefrom another optical recording medium. In addition, the opticalrecording/reproducing apparatus may record onto or reproduce from oneoptical information recording medium and record onto and reproduce fromanother optical information recording medium. The term “reproduce”includes merely reading information.

Since each of the objective lenses of [1] and [2] has at least oneaspheric surface, it is possible to satisfactorily correct variousaberrations. In addition, since each objective lens is formed of asingle lens, it is possible to contribute to reduction in weight oflens. As a result, it is possible to reduce a weight of lens, whichenables to drive the lens at a higher speed in focusing and trackingcontrols.

Furthermore, since the objective lenses of [1] and [2] satisfy theconditional expression (1), a lens material having a low refractiveindex can be used, and a specific gravity of a lens material, which hasa tendency to increase in specific gravity in accordance with anincrease in refractive index, can be set small. Therefore, it ispossible to achieve reduction in weight of the lens. Thereby, it ispossible to drive the lens at a high speed in the focusing and trackingcontrols.

Further, since the objective lens of [1] and [2] satisfy the conditionalexpression (2), it is possible to prevent an excessive increase incurvature of the optical recording medium side surface. Thereby, it ispossible to reduce spherical aberration caused by the optical recordingmedium side surface. On the other hand, it is also possible to preventan excessive increase in curvature of the light source side surface. Asa result, it is possible to increase a working distance of the objectivelens.

In addition, since the objective lens of [1] satisfies the conditionalexpression (3), it is possible to obtain favorable off-axischaracteristics by satisfying the lower limit range thereof. On theother hand, by satisfying the upper limit range thereof, it is possibleto decrease a weight and a volume of the objective lens. As a result, itis possible to drive the objective lens at a high speed during control.

Further, since the objective lens of [2] satisfies the conditionalexpression (4), it is possible to decrease a weight and a volume of theobjective lens by satisfying the lower limit range thereof. Accordingly,it is possible to drive the objective lens at a high speed duringcontrol. On the other hand, by satisfying the upper limit range, it ispossible to increase a working distance of the objective lens.

According to the above configurations, it is possible to provide anoptical pickup device having the objective lens, which has theabove-mentioned advantages, and the optical recording/reproducingapparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view schematically illustrating an objective lensaccording to Example 1 of the invention.

FIG. 2 is a sectional view schematically illustrating an objective lensaccording to Example 2 of the invention.

FIG. 3 is a sectional view schematically illustrating an objective lensaccording to Example 3 of the invention.

FIG. 4 is a sectional view schematically illustrating an objective lensaccording to Example 4 of the invention.

FIG. 5 is a schematic configuration view illustrating an optical pickupdevice according to an embodiment of the invention.

FIG. 6 is a wavefront aberration view illustrating the objective lensaccording to Example 1 of the invention.

FIG. 7 is a wavefront aberration view illustrating the objective lensaccording to Example 2 of the invention.

FIG. 8 is a wavefront aberration view illustrating the objective lensaccording to Example 3 of the invention.

FIG. 9 is a wavefront aberration view illustrating the objective lensaccording to Example 4 of the invention.

FIG. 10 is a view illustrating relationship between wavefront aberrationand a thickness of a protection layer of the objective lens according toExample 1 of the invention.

FIG. 11 is a view illustrating relationship between wavefront aberrationand a thickness of a protection layer of the objective lens according toExample 2 of the invention.

FIG. 12 is a view illustrating relationship between wavefront aberrationand a thickness of a protection layer of the objective lens according toExample 3 of the invention.

FIG. 13 is a view illustrating relationship between wavefront aberrationand a thickness of a protection layer of the objective lens according toExample 4 of the invention.

FIG. 14A is a view for explaining an outer diameter Do of the objectivelens according to an embodiment of the invention.

FIG. 14B is a view for explaining the outer diameter Do of the objectivelens according to an embodiment of the invention.

FIG. 14C is a view for explaining the outer diameter Do of the objectivelens according to an embodiment of the invention.

FIG. 14D is a view for explaining the outer diameter Do of the objectivelens according to an embodiment of the invention.

FIG. 15 is a schematic perspective view illustrating an opticalrecording/reproducing apparatus according to an embodiment of theinvention.

FIG. 16 shows a relationship between the effective diameter De and theouter diameter Do of the objective lens.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereinafter, embodiments of the invention will be described withreference to the accompanying drawings.

FIGS. 1 to 4 are sectional diagrams illustrating the configurations ofobjective lenses according to the embodiments of the invention, andcorrespond to objective lenses of Examples 1 to 4. FIG. 5 is a diagramillustrating the configuration of an optical pickup device according toan embodiment of the invention.

First, referring to FIG. 5, the optical pickup device 10 according tothe embodiment of the invention will be described. Subsequently, theobjective lenses according to the embodiments of the invention will bedescribed in detail.

The optical pickup device 10 includes an objective lens 8 according tothe embodiment of the invention, a semiconductor laser 1 serving as alight source, a half mirror 6 that is disposed obliquely at an angle of45 degrees with respect to light emitted from the semiconductor laser 1,a collimator lens 7, and a photodiode 13. The optical pickup device 10performs at least one of operations of recording and reproducinginformation by having the objective lens 8 converge the light, which isemitted from the semiconductor laser 1, onto an optical recording medium9 which the information is to be recorded onto or reproduced from.

In the following description of the embodiment, an example in which theblu-ray disk is used as the optical recording medium 9 will bedescribed, but the invention is not limited thereto. According to therecent specification of the blu-ray disk, an NA is 0.85, a wavelength ofused light is 405 nm, and a thickness of a protection layer is 0.1 mm.However, a single layer disk and a double layer disk are different inposition of an optical recording layer. That is, the optical recordinglayer of the single layer disk is disposed 0.100 mm from the surfacethereof, and the optical recording layers of the double layer disk aredisposed 0.075 mm and 0.100 mm from the surface thereof.

The optical pickup device 10 includes a lens holder 15 for holding theobjective lens 8, and a mask 16 disposed on the light source side of theobjective lens 8. The lens holder 15 is fixedly attached to a part of aperipheral side surface of a flange portion of the objective lens 8 anda part of a light source side flat surface of the flange portion of theobjective lens 8, and is integrally formed with an actuator not shown inthe drawing. A servo mechanism including this actuator performs trackingand focusing operations of the objective lens 8. The mask 16 has apredetermined aperture diameter so as to obtain a desired NA on theoptical recording medium side of the objective lens 8.

The semiconductor laser 1 is a light source for emitting blue laserlight having a wavelength of 405 nm. Furthermore, a light sourceemitting a laser light in a wavelength range of 400.0 nm to 410.0 nm maybe employed in the embodiment of the invention.

The collimator lens 7, which is schematically shown in FIG. 5, is notlimited to the single element configuration, and may be formed of plurallenses. The collimator lens 7 is adapted to make the light emitted fromthe semiconductor laser 1 incident on a light source side surface of theobjective lens 8 in a state of parallel light rays.

A photodiode whose light receiving section is divided into four may beused as the photodiode 13.

Inside of the optical recording medium 9, pits (each of which is notnecessary to have a physically concave shape) carrying signalinformation are arranged in a track shape, and an optical recordinglayer 9 b which information can be recorded into and reproduced from isformed. A protection layer 9 a transparent to light of the light sourceis formed between the objective lens side surface of the opticalrecording medium 9 and the optical recording layer 9 b. A thickness ofthe protection layer 9 a of the optical recording medium 9 is 0.1 mm.Either a single layer disk which has only a single optical recordinglayer or a double layer disk which has two optical recording layers maybe employed as the optical recording medium 9 in this embodiments of theinvention. FIG. 5 schematically shows the optical recording layer 9 b,but the optical recording medium 9 is not limited to the single layerdisk.

In the optical pickup device 10 having such a configuration, the laserlight emitted from the semiconductor laser 1 is reflected by the halfmirror 6 at an angle of 90 degrees with respect to an optical path, issubstantially collimated by the collimator lens 7, and is converged bythe refraction effect of the objective lens 8. The refraction effectcauses the light exiting from the optical recording medium side surfaceof the objective lens 8 to be satisfactorily converged onto the opticalrecording layer 9 b which information is to be recorded onto orreproduced from. At this time, the objective lens 8 is driven by theabove-mentioned servo mechanism, and thereby, the converged light can belocated on the optical recording layer 9 b of the optical recordingmedium 9.

The reflected light from the optical recording layer 9 b is transmittedthrough the objective lens 8, the collimator lens 7 and the half mirror6 in a state where the light carries signal information, and is incidenton the photodiode 13. The photodiode 13 outputs electric signals inresponse to respective light receiving amounts of the four segment partsof the light receiving section. Based on the electric signals, acalculation unit which is not shown performs a predeterminedcalculation, thereby obtaining a data signal, a focus error signal and atracking error signal.

The half mirror 6 is inserted to have at an angle of 45 degrees withrespect to the optical path of the returning light from the opticalrecording medium 9. Therefore, the light beams transmitted through thehalf mirror 6 have astigmatism. Thereby, an amount of the focus error isdetermined in accordance with a shape of a beam spot of the returninglight on the four-divided photodiode 13. Generally, a method that causesastigmatism by use of a cylindrical lens to perform light detection hasbeen known, and the half mirror 6 of this embodiment has the samefunction as the cylindrical lens in that method. In addition, it may bepossible to detect a tracking error based on three beams separated byinserting an optical element such as a grating between the semiconductorlaser 1 and the half mirror 6.

Hereinafter, the objective lens 8 according to the embodiments will bedescribed with reference to FIGS. 1 to 4. The objective lens 8 is formedof a single lens. Since the objective lens is formed of a single lens,it is not necessary to adjust alignment between lenses at the time ofassembly. As a result, it is possible to improve production efficiencyand reduce costs.

Furthermore, at least one surface of the objective lens 8 is an asphericsurface, and it is preferable that both surfaces thereof be asphericsurfaces. It is more preferable that the aspheric surface be an asphericsurface that is rotationally symmetric and is represented by thefollowing aspherical expression. By forming such a rotationallysymmetric aspheric surface, it is possible to satisfactorily correctvarious aberrations such as spherical aberration and comatic aberration.Thus, it is possible to surely perform the focusing and trackingoperations and satisfactorily perform the recording and reproducingoperations.

Further, in the objective lens 8, it is preferable that the opticalrecording medium side surface of the objective lens 8 be formed in aconvex shape near the optical axis, and be formed in a concave shape ata predetermined position distant from the optical axis. By forming theobjective lens in such a shape, it is possible to satisfactorily correctspherical aberration, and the objective lens is configured tosatisfactorily perform recording and reproducing. In addition, it ispreferable that the shape of the aspheric surface formed on theobjective lens 8 be appropriately set to satisfactorily correctaberrations so as to converge the used light onto the optical recordinglayer 9 b.

$Z = {\frac{C \times Y^{2}}{1 + \sqrt{1 - {K \times C^{2} \times Y^{2}}}} + {\sum\limits_{i = 4}^{20}{A_{i}{Y}^{i}}}}$where Z denotes a length of a perpendicular drawn from a point on anaspheric surface, that is distant Y from the optical axis, to atangential plane (plane perpendicular to the optical axis) for theaspheric vertex,

Y denotes a distance from the optical axis,

C denotes a curvature of the aspheric surface near the optical axis,

K denotes a conic coefficient, and

A_(i) denotes an aspheric coefficient (i=4 to 20).

Further, the objective lens 8 is configured to satisfy the followingconditional expressions (1) to (3) or the following conditionalexpressions (1), (2), and (4):N≦1.75  (1)0.5<f/f ₁<0.6  (2)0.8<d/(NA·De)<1.0  (3)0.3<d/Do<0.8  (4)where

N denotes a refractive index of the objective lens,

f denotes a focal length (mm) of the objective lens,

f₁ denotes a focal length (mm) of the light source side surface of theobjective lens,

d denotes a thickness (mm) of the objective lens on an optical axis,

NA denotes a numerical aperture of the objective lens on the opticalrecording medium side,

De denotes an effective aperture (mm) of the light source side surfaceof the objective lens, and

Do denotes an outer diameter (mm) of the objective lens.

The conditional expression (1) defines a value of refractive index N ofa material constituting the objective lens 8. By satisfying theconditional expression (1), a specific gravity of a lens material, whichhas a tendency to increase in specific gravity in accordance with anincrease in refractive index, can be set small. As described above,generally, a specific gravity of a material having a refractive index of1.85 becomes 1.5 times as large as a specific gravity of a materialhaving a refractive index of 1.60, and a weight thereof becomes large inaccordance therewith. Accordingly, generally, when a material having arefractive index of about 1.8 or more, that is, an ultrahigh refractiveindex, it becomes difficult to drive a lens at a high speed in focusingand tracking controls. Since a refractive index N of the material ofthis embodiment is equal to or less than 1.75 so as to satisfy theconditional expression (1), it is possible to reduce a weight of lens.As a result, it is possible to drive the lens at a high speed infocusing and tracking controls.

In the objective lens 8 of this embodiment, when a material having arefractive index that satisfies the conditional expression (1) isselected, it is preferable to form the objective lens in a biconvexshape near the optical axis as the example shown in FIG. 1. When amaterial having a refractive index of 1.75 or less is used, the biconvexlens shape has a larger effect in correcting spherical aberration than apositive meniscus lens shape. In addition, when a material having arefractive index of 1.69 or less is used, this effect in correctingspherical aberration becomes larger.

The conditional expression (2) defines a ratio of the focal length f ofthe objective lens 8 to the focal length f₁ of the light source sidesurface of the objective lens 8. By satisfying the lower limit range ofthe conditional expression (2), it is possible to prevent that acurvature of the optical recording medium side surface of the objectivelens 8 becomes excessively large. Therefore, it is possible to decreasespherical aberration caused by the optical recording medium side surfaceof the objective lens 8. Furthermore, by satisfying the upper limitrange of the conditional expression (2), it is possible to prevent thatcurvature of the light source side surface of the objective lens 8becomes excessively large. As a result, it is possible to increase aworking distance thereof.

The conditional expression (3) defines a range of a ratio of thethickness d of the objective lens 8 on the optical axis to a valueobtained by multiplying the effective aperture De of the objective lens8 by the numerical aperture NA of the objective lens 8 on the opticalrecording medium side. By satisfying the lower limit range of theconditional expression (3), it is possible to obtain favorable off-axischaracteristics. Also, by satisfying the upper limit range of theconditional expression (3), it is possible to suppress the thickness ofthe objective lens 8 and decrease a weight and a volume thereof. As aresult, it is possible to drive the objective lens 8 at a high speedduring control.

The conditional expression (4) defines a range of a ratio of thethickness d of the objective lens 8 on the optical axis to the outerdiameter Do of the objective lens 8. By satisfying the lower limit rangeof the conditional expression (4), it is possible to suppress the outerdiameter of the objective lens 8 and decrease a weight and a volumethereof. As a result, it is possible to drive the objective lens 8 at ahigh speed during control. By satisfying the upper limit range of theconditional expression (4), it is possible to suppress the thickness ofthe objective lens 8 so that a working distance of the objective lens 8can be increased.

Here, the outer diameter Do of the objective lens 8 used in theconditional expression (4) is defined as a diameter of an arcconstituting an outer shape of a lens in consideration of the outershape of the lens including a flange portion in a plane perpendicular tothe optical axis AX. The Do will be described with reference to FIGS.14A to 14D. Each solid line shown in FIGS. 14A to 14D represents theouter shape of the objective lens having the flange portion when viewedfrom the optical axis direction. In FIGS. 14A to 14D, each dotted linedenotes an outer diameter of an area that has a curved lens surface inthe light source side surface, and the so-called flange portion existsbetween the dotted line and the solid line representing the outer shapeof the objective lens 8. The shape of the flange portion when viewedfrom the optical axis direction may not be a perfect ring shape as shownin FIG. 14A, and may be a shape a part of which is not a ring shape asshown in FIGS. 14B to 14D. However, normally, a half or more of theouter shape of the flange portion is formed as an arc. Therefore, evenif the whole area of the outer shape of the objective lens 8 is not anarc, a diameter of the arc constituting the outer shape of the objectivelens 8 is defined as the outer diameter Do of the objective lens 8.

The “effective aperture” of the objective lens is, for example, definedas that “a maximum diameter of an area, on a surface of the objectivelens, which should collect light rays and energy passing through theobjective lens and transmit the light rays and energy to an opticalrecording medium, a final detector or the like, wherein the light rays(effective light rays) pass through the area.” In specific opticalsystems, the area through which the effective light rays pass changesdepending on an aperture diameter of an aperture diaphragm and the like.However, the “maximum diameter of the area through which the effectivelight rays pass” are determined by the design of the objective lensitself. FIG. 16 shows a relationship between the effective diameter Deand the outer diameter Do of the objective lens 8. In FIG. 16, thedouble-dashed lines represent trajectories of light rays that passthrough the outer ends of the effective diameter De of the objectivelens 8.

By satisfying the following conditional expression (4-1) instead of theconditional expression (4), it is possible to make better theabove-mentioned effect of the conditional expression (4).0.4<d/Do<0.6  (4-1)

In the objective lens 8 of this embodiment, it is preferable to satisfythe following conditional expression (5) or (6).0.50<g/d<0.80  (5)25<v  (6)where g denotes a distance (mm) from a tangential plane that isperpendicular to the optical axis AX and tangent to a vertex of thelight source side surface to a center of gravity of the objective lens8, and

v denotes an Abbe number of the objective lens 8 at the d-line.

The conditional expression (5) defines an appropriate range of thecenter of gravity G of the objective lens 8 in consideration of the casewhere the objective lens 8 is mounted on the optical pickup device isdriven. Specifically, in the conditional expression (5), in order tosecure stable operations of the optical pickup device, the followingconditions are considered. The center of gravity G of the objective lensshould not be set at position excessively distant from the centralposition thereof in a thickness on the optical axis. The objective lensfor an optical pickup device is formed in such a shape that a convexsurface with a large curvature is directed toward the light source.Because of these conditions, unless the center of gravity G of theobjective lens is set in a predetermined range closer to the opticalrecording medium than the central position thereof in the thickness onthe optical axis, the shape of the objective lens renders the operationof the optical pickup device unstable, and restricts degree of freedomin lens design severely. In consideration of a range capable of solvingthese problems, the conditional expression is defined. As a result, bysatisfying the conditional expression (5), it is possible to secure astable operation of the optical pickup device without loss of degree offreedom in lens design.

The conditional expression (6) defines that the Abbe number v of amaterial of the lens is greater than 25. By setting the range in theconditional expression (6), it is possible to prevent that chromaticaberration becomes excessively large. Accordingly, even if a wavelengthof light emitted from the light source is fluctuated, it is possible toprevent problems that an image point moves, a beam spot blurs, and soon. In order to prevent that chromatic aberration becomes excessivelylarge, it is preferable that the Abbe number v of the material of thelens be not less than 35.

In the objective lens 8 of this embodiment, it is preferable that an NAof the lens on the optical recording medium side be not less than 0.70and not more than 0.98. By setting the NA to be in the range of 0.70 to0.98, it is possible to reduce a diameter of a spot where light isconverged onto the optical recording layer 9 b of the optical recordingmedium 9. As a result, it is possible to perform recording andreproducing with high density. It is also possible to record onto andreproduce from, with higher density, a new optical recoding medium whichwill be developed in future. In the objective lens 8 of this embodiment,it is preferable that an NA be not less than 0.85. In this case, theabove-mentioned advantages can be obtained, and the objective lensbecomes compatible with an NA prescribed by the present specificationsof the blu-ray disk.

It is preferable for the objective lens 8 of this embodiment tosatisfactorily converge light onto the optical recording medium so thatinformation can be recorded and reproduced when the light emitted fromthe light source is not less than 400.0 nm and not more than 410.0 nm.Although a wavelength prescribed in the specifications of the blu-raydisk is 405 nm, an output wavelength of a semiconductor laser is notalways stabilized, and output wavelengths of individual semiconductorlasers are different. Accordingly, it is preferable to employ theobjective lens capable of dealing with those mentioned above.

In the objective lens 8 of this embodiment, when a thickness of theprotection layer 9 a of the optical recording medium 9 is in not lessthan 0.075 mm and not more than 0.1 mm, it is preferable that a fineimage be formed at a position in that range of thickness.

Furthermore, in the objective lens 8 of the embodiment, when a RMS ofwavefront aberration is set to be minimized at a position distant t1(mm) from a surface of the optical recording medium toward the inside ofthe optical recording medium, it is preferable that the t1 be not lessthan 0.075 mm and not more than 0.1 mm.

In a double layer disk, the condition of t1 is a condition for formingfine images on positions of both recording layers when the recordinglayers are provided at positions distant 0.075 mm and 0.100 mm from theobjective lens side surface of the recording medium toward the inside ofthe medium, respectively. When t1 is outside that range, greatdifference is caused between image formation states of the two surfacesof the recording layers since an image formed at one of the tworecording layers is fine but an image formed at the other recordinglayer is deteriorated.

Further, the condition of t1 is a condition for minimizing load on theoptical pickup apparatus as small as possible, such as a lens movementdistance for aberration correction when the optical pickup device has anaberration correction device that, for example, moves the lens in theoptical axis direction for adjustment, in order to improve the state ofimage formation on the recording surface positions. With such aconfiguration, it is possible to shorten a time for aberrationcorrection.

In the objective lens 8 of this embodiment, a smaller mass of the lensis more advantageous in order to reduce load on the actuator forperforming a focus control and a tracking control at the time ofrecording onto or reproducing from a high density recording medium. Forexample, it is preferable that a mass of the lens be not more than 0.5grams. It is more preferable that a mass of the lens be not more than0.1 gram, and it is further preferable that a mass of the lens be notmore than 0.02 grams.

The objective lens 8 may be made of plastic. Exemplary advantages ofusing plastic materials include reduction in manufacturing costs,reduction in weight, fast recording and reading because of the reductionin weight, and improvement in processability of a mold.

Alternatively, the objective lens 8 may be made of glass. Exemplaryadvantages of using glass materials include excellent resistance totemperature and humidity, and ease of acquisition of materials which hasless deterioration in transmittance even if short wavelength light isapplied for a long time.

Hereinafter, the objective lens of the embodiments of the invention willbe described in detail with reference to the following examples.

EXAMPLES

FIGS. 1 to 4 are lens sectional views of the objective lenses accordingto Examples 1 to 4. FIGS. 6 to 9 are on-axis wavefront aberration views.FIGS. 10 to 13 show variations of wavefront aberrations relative tothicknesses of a protection layer. FIGS. 1 to 4 show the protectionlayer 9 a and the conceptual recording layer 9 b of the opticalrecording medium 9.

Example 1

The objective lens 8 according to Example 1 is formed of a single lensmade of glass. As shown in FIG. 1, the light source side surface 8 a andthe optical recording medium side surface 8 b are convex surfaces nearthe optical axis, and the light source side surface 8 a is the convexsurface having a larger curvature than the optical recording medium sidesurface 8 b. Furthermore, both the surfaces of the objective lens 8according to this example are aspheric surfaces each of which isrotationally symmetric.

In the upper part of the following Table 1, surface numbers are assignedso as to sequentially increase as it gets closer to the opticalrecording medium side from the light source side when the light sourceside surface is numbered as 1. The upper part thereof also shows thefollowing items as lens data of the objective lens 8 according toExample 1: radiuses of curvature R (mm) of surfaces; on-axis surfacespacings D (mm); and refractive indices N at a wavelength of the usedlight. In the column of radius of curvature, if the correspondingsurface is aspheric, its radius of curvature is described as asphericsurface. Furthermore, the lens data includes data of the protectionlayer 9 a of the optical recording medium 9.

Furthermore, the middle part of the following Table 1 shows thefollowing items as aspheric surface data of the objective lens 8according to Example 1: a curvature C near the optical axis; asphericcoefficients K; and A₄ to A₂₀ (only even-order coefficients) in theaspheric expression mentioned above. For C in the aspheric surface data,if the aspheric surface is convex toward the light source side, thealgebraic sign of C is positive; if the aspheric surface is convextoward the optical recording medium side, the algebraic sign of C isnegative. “E−j” (j: integer) used in the numeral values of K and A₄ toA₂₀ means “×10^(−j)”, and “E+0” means “×10⁰”. In the middle part ofTable 1, the light source side surface 8 a of the objective lens isdesignated as a first surface, and the optical recording medium sidesurface 8 b is designated as a second surface.

Furthermore, the lower part of the following Table 1 shows various dataof the objective lens 8 according to Example 1. A wavelength λ denotes awavelength of the used light (design wavelength), and numerical valuesof the various data shown in the lower part of Table 1, except for theAbbe number, is based on the wavelength of the used light. The Abbenumber is measured at the d-line. The working distance WD is a distancefrom a vertex of the optical recording medium side surface to theobjective lens side surface of the optical recording medium. Thethickness t1 of the protection layer is a thickness of the protectionlayer from the objective lens side surface of the optical recordingmedium to a position at which an RMS of wavefront aberration takes theminimum value. The on-axis wavefront aberration is a value at theposition of the thickness t1. The off-axis wavefront aberration is avalue at a total angle of view of 1 degree. Furthermore, meaning of theabove-mentioned data is the same in Examples 2 to 4.

TABLE 1 Example 1 <Lens Data> Radius of curvature R Surface spacingRefractive Surface (mm) D (mm) index N 1 Aspheric 1.447 1.605 2 Aspheric0.343 1.000 3 ∞ 0.0875 1.619 4 ∞ <Aspheric Surface Data> 1st surface 2ndsurface C 1.214757025 −0.563541950 K 2.324847420E−01 1.071518972E+00 A₄6.523776124E−02 1.636467661E+00 A₆ 6.801866828E−02 −6.320648469E+00 A₈−1.574817411E−01 1.597136639E+01 A₁₀ 5.795272288E−01 −2.781930450E+01A₁₂ −9.837283446E−01 3.224527681E+01 A₁₄ 8.968530874E−01−2.262496781E+01 A₁₆ −3.536943856E−01 7.291371700E+00 A₁₈0.000000000E+00 0.000000000E+00 A₂₀ 0.000000000E+00 0.000000000E+00<Various Data> Wavelength λ (nm) 405.00 NA 0.850 Focal length f (mm)1.1760 Back focal length bf (mm) 0.3966 Refractive index N 1.605 Focallength of light source side surface f₁ 2.183 Thickness of lens onoptical axis d (mm) 1.447 Effective aperture D_(e) 2.00 Outer diameterD_(O) 3.00 Working distance WD (mm) 0.343 g (mm) 0.862 Mass (grams)0.016 Abbe number ν 59.96 t1 (mm) 0.0875 On-axis wavefront aberration(λRMS) 0.0009 Off-axis wavefront aberration (λRMS) 0.017

The objective lens according to Example 1 can achieve a large NA of 0.85when a wavelength λ of the used light is 405.0 nm. In addition, as shownin FIG. 6 and the value of the on-axis wavefront aberration in Table 1,the wavefront aberration is good, and the lens can satisfactorilyconverge the used light onto the optical recording layer 9 b of theoptical recording medium 9. Further, the objective lens according toExample 1 is configured to secure the sufficient working distance of0.343 mm as shown in Table 1. Also, since the off-axis wavefrontaberration is 0.017 λRMS as shown in Table 1, the lens according to theExample 1 can achieve excellent image height characteristic. Also, thelens according to Example 1 can achieve reduction in weight, that is, amass thereof is 0.016 grams, as shown in Table 1.

In the objective lens 8 according to Example 1, the optical recordingmedium 9 is assumed as a double layer disk, and in order to deal withthe double layer disk, aberration is set so that aberrations arecorrected well at an intermediate position between the two recordinglayers. Consequently, the objective lens according to Example 1 isconfigured to minimize a wavefront aberration at the position distant0.0875 mm from the surface of the optical recording medium 9, and theon-axis wavefront aberration at this position is 0.0009 λRMS, as shownin FIG. 10.

Example 2

The objective lens 8 according to Example 2 is formed of a single lensmade of glass. As shown in FIG. 2, the light source side surface 8 a andthe optical recording medium side surface 8 b are convex surfaces nearthe optical axis, and the light source side surface 8 a is the convexsurface having a larger curvature than the optical recording medium sidesurface 8 b. Furthermore, both the surfaces of the objective lens 8according to this example are aspheric surfaces each of which isrotationally symmetric.

As for data according to Example 2, the upper part of the followingTable 2 shows lens data, the middle part thereof shows aspheric surfacedata, and the lower part thereof shows various data.

TABLE 2 Example 2 <Lens Data> Radius of curvature R Surface spacingRefractive Surface (mm) D (mm) index N 1 Aspheric 1.447 1.605 2 Aspheric0.337 1.000 3 ∞ 0.1000 1.619 4 ∞ <Aspheric Surface Data> 1st surface 2ndsurface C 1.211757025 −0.569673325 K 2.231072347E−01 9.067227885E−01 A₄6.679499460E−02 1.623953466E+00 A₆ 7.115742222E−02 −6.252104914E+00 A₈−1.718328591E−01 1.583893475E+01 A₁₀ 6.174314551E−01 −2.792750177E+01A₁₂ −1.038716835E+00 3.304041599E+01 A₁₄ 9.369370245E−01−2.378315428E+01 A₁₆ −3.649461167E−01 7.870630127E+00 A₁₈0.000000000E+00 0.000000000E+00 A₂₀ 0.000000000E+00 0.000000000E+00<Various Data> Wavelength λ (nm) 405.00 NA 0.850 Focal length f (mm)1.1760 Back focal length bf (mm) 0.3985 Refractive index N 1.605 Focallength of light source side surface f₁ 2.189 Thickness of lens onoptical axis d (mm) 1.447 Effective aperture D_(e) 2.00 Outer diameterD_(O) 3.00 Working distance WD (mm) 0.337 g (mm) 0.861 Mass (grams)0.014 Abbe number ν 59.96 t1 (mm) 0.1000 On-axis wavefront aberration(λRMS) 0.0010 Off-axis wavefront aberration (λRMS) 0.017

The objective lens according to Example 2 can achieve a large NA of 0.85when a wavelength λ of the used light is 405.0 nm. In addition, as shownin FIG. 7 and the value of on-axis wavefront aberration in Table 2, thewavefront aberration is good, and the lens can satisfactorily convergethe used light onto the optical recording layer 9 b of the opticalrecording medium 9. Further, the objective lens according to Example 2is configured to secure a sufficient working distance of 0.337 mm asshown in Table 2. Also, since the off-axis wavefront aberration is 0.017λRMS as shown in Table 2, the lens according to the Example 2 canachieve excellent image height characteristics. Also, the lens accordingto the Example 2 can achieve reduction in weight, that is, a massthereof is 0.014 grams, as shown in Table 2.

In the objective lens 8 according to Example 2, the optical recordingmedium 9 is assumed as a single layer disk. The objective lens accordingto Example 2 is configured to minimize a wavefront aberration at theposition distant 0.1 mm from the surface of the optical recording medium9, and the on-axis wavefront aberration at this position is 0.0010 λRMS,as shown in FIG. 11.

Example 3

The objective lens 8 according to Example 3 is formed of a single lensmade of plastic. As shown in FIG. 3, the light source side surface 8 aand the optical recording medium side surface 8 b are convex surfacesnear the optical axis, and the light source side surface 8 a is theconvex surface having a larger curvature than the optical recordingmedium side surface 8 b. Furthermore, both the surfaces of the objectivelens 8 according to this example are aspheric surfaces each of which isrotationally symmetric. Furthermore, the optical recording medium sidesurface of the objective lens 8 of this example is formed in a convexshape near the optical axis and is formed in a concave shape at apredetermined position distant from the optical axis AX.

As for data according to Example 3, the upper part of the followingTable 3 shows lens data, the middle part thereof shows aspheric surfacedata, and the lower part thereof shows various data.

TABLE 3 Example 3 <Lens Data> Radius of curvature R Surface spacingRefractive Surface (mm) D (mm) index N 1 Aspheric 2.253 1.525 2 Aspheric0.502 1.000 3 ∞ 0.1000 1.618 4 ∞ <Aspheric Surface Data> 1st surface 2ndsurface C 0.877164470 −0.630277255 K 4.496547921E−02 1.524018153E+00 A₄3.823031376E−02 8.778900757E−01 A₆ 3.518063967E−03 −1.557286522E+00 A₈1.858860704E−02 1.879314685E+00 A₁₀ −2.594878831E−02 −1.370939101E+00A₁₂ 2.385687100E−02 1.300798787E+00 A₁₄ −1.122473371E−02−2.270369528E+00 A₁₆ 2.344349476E−03 2.611797039E+00 A₁₈−8.484603101E−05 −1.482269017E+00 A₂₀ −8.752092239E−06 3.299435201E−01<Various Data> Wavelength λ (nm) 408.00 NA 0.850 Focal length f (mm)1.7654 Back focal length bf (mm) 0.5640 Refractive index N 1.525 Focallength of light source side surface f₁ 3.311 Thickness of lens onoptical axis d (mm) 2.253 Effective aperture D_(e) 3.00 Outer diameterD_(O) 4.00 Working distance WD (mm) 0.502 g (mm) 1.333 Mass (grams)0.015 Abbe number ν 55.91 t1 (mm) 0.1000 On-axis wavefront aberration(λRMS) 0.0019 Off-axis wavefront aberration (λRMS) 0.029

The objective lens according to Example 3 can achieve a large NA of 0.85when a wavelength λ of the used light is 408.0 nm. In addition, as shownin FIG. 8 and the value of the on-axis wavefront aberration in Table 3,the wavefront aberration is good, and the lens can satisfactorilyconverge the used light onto the optical recording layer 9 b of theoptical recording medium 9. Further, the objective lens according toExample 3 is configured to secure a sufficient working distance of 0.502mm as shown in Table 3. Also, since an off-axis wavefront aberration is0.029 λRMS as shown in Table 3, the objective lens according to Example3 can achieve excellent image height characteristics. Also, theobjective lens according to the Example 3 can achieve reduction inweight, that is, a mass thereof is 0.015 grams, as shown in Table 3.

In the objective lens 8 according to Example 3, the optical recordingmedium 9 is assumed as a single layer disk. The objective lens accordingto Example 3 is configured to minimize a wavefront aberration at theposition distant 0.1 mm from the surface of the optical recording medium9, and the on-axis wavefront aberration at this position is 0.0019 λRMS,as shown in FIG. 12.

Example 4

The objective lens 8 according to Example 4 is formed of a single lensmade of glass. As shown in FIG. 4, the light source side surface 8 a andthe optical recording medium side surface 8 b are convex surfaces nearthe optical axis, and the light source side surface 8 a is the convexsurface having a larger curvature than the optical recording medium sidesurface 8 b. Furthermore, both the surfaces of the objective lens 8according to this example are aspheric surfaces each of which isrotationally symmetric. In addition, the optical recording medium sidesurface of the objective lens 8 according to this example is the convexshape near the optical axis, and is formed in a concave shape at apredetermined position distant from the optical axis AX.

As for data according to Example 4, the upper part of the followingTable 4 shows lens data, the middle part thereof shows aspheric surfacedata, and the lower part thereof shows various data.

TABLE 4 Example 4 <Lens Data> Radius of curvature R Surface spacingRefractive Surface (mm) D (mm) index N 1 Aspheric 2.580 1.605 2 Aspheric0.721 1.000 3 ∞ 0.0875 1.619 4 ∞ <Aspheric Surface Data> 1st surface 2ndsurface C 0.665963094 −0.241274788 K 1.485482017E−01 −1.592117057E+00 A₄1.458997407E−02 2.062426083E−01 A₆ 1.248696532E−03 −1.965367777E−01 A₈2.034812605E−03 1.092415726E−01 A₁₀ −1.289837288E−03 −2.610313432E−02A₁₂ 6.134250622E−04 −4.565082721E−03 A₁₄ −1.409898547E−044.109929533E−03 A₁₆ 1.313656939E−05 −6.899202346E−04 A₁₈ 0.000000000E+000.000000000E+00 A₂₀ 0.000000000E+00 0.000000000E+00 <Various Data>Wavelength λ (nm) 405.00 NA 0.850 Focal length f (mm) 2.2000 Back focallength bf (mm) 0.7747 Refractive index N 1.605 Focal length of lightsource side surface f₁ 3.982 Thickness of lens on optical axis d (mm)2.580 Effective aperture D_(e) 3.74 Outer diameter D_(O) 5.00 Workingdistance WD (mm) 0.721 g (mm) 1.539 Mass (grams) 0.073 Abbe number ν59.96 t1 (mm) 0.0875 On-axis wavefront aberration (λRMS) 0.0016 Off-axiswavefront aberration (λRMS) 0.029

The objective lens according to Example 4 can achieve a large NA of 0.85when a wavelength λ of the used light is 405.0 nm. In addition, as shownin FIG. 9 and the value of on-axis wavefront aberration in Table 4, thewavefront aberration is good, and the lens can satisfactorily convergethe used light onto the optical recording layer 9 b of the opticalrecording medium 9. Further, the objective lens according to Example 4is configured to secure a sufficient working distance of 0.721 mm asshown in Table 4. Also since an off-axis wavefront aberration is 0.029λRMS as shown in Table 4, the objective lens according to Example 4 canachieve excellent image height characteristics. Also, the objective lensaccording to Example 4 can achieve reduction in weight, that is, a massthereof is 0.073 grams, as shown in Table 4.

In the objective lens 8 according to Example 4, the optical recordingmedium 9 is assumed as a double layer disk, and in order to deal withthe double layer disk, aberration is set to take a good value at anintermediate position between the two recording layers. Consequently,the objective lens 8 according to Example 4 is configured to minimize awavefront aberration at the position distant 0.0875 mm from the surfaceof the optical recording medium 9, and the on-axis wavefront aberrationat this position is 0.0016 λRMS, as shown in FIG. 13.

Table 5 shows values corresponding to the conditional expressions (1) to(6) in Examples 1 to 4. As shown in Table 5, all the Examples 1 to 4satisfy the conditional expressions (1) to (6) (including (4-1)).

TABLE 5 Cond. (2) Cond. (3) Cond. (4) Cond. (5) Example Cond. (1) N f/f₁d/(NA · De) d/D_(o) g/d Cond. (6) ν 1 1.60532 0.539 0.852 0.482 0.59659.96 2 1.60532 0.537 0.852 0.482 0.595 59.96 3 1.52522 0.533 0.8830.563 0.592 55.91 4 1.60532 0.552 0.812 0.516 0.597 59.96

Next, an optical recording/reproducing apparatus according to anembodiment of the invention will be described with reference to FIG. 15.FIG. 15 is a schematic perspective view illustrating an opticalrecording/reproducing apparatus 30 according to this embodiment of theinvention. The optical recording/reproducing apparatus 30 has theoptical pickup device 32 according to the embodiment of the inventiontherein. The front side of the apparatus 30 is provided with aninsertion portion 34 for inserting an optical recording medium, andoperation buttons 35 a, 35 b, and 35 c for performing various operationssuch as recording, reproducing, and pause. In addition, since theoptical recording/reproducing apparatus 30 has the objective lensaccording to the embodiment of the invention, it is possible tosatisfactorily record onto or reproduce from an optical recording mediumsuch as a blu-ray disk.

Although the invention has been described through the embodiments andexamples as above, the invention is not limited thereto, and may bemodified in various manner. For example, values of radiuses ofcurvature, aspheric coefficients, on-axis surface spacings, andrefractive indices of lens components are not limited to the valuesshown in the numerical examples, and may have different values.

For example, the objective lens according to the invention is notlimited to the configuration in which both of the light source sidesurface and the optical recording medium side surface are formed asrotationally symmetric aspheric surfaces similarly to the examples. Whenat least one surface thereof (in a case of one surface, it is preferableto select the light source side surface) is an aspheric surface, theother surface may be a flat surface or a spherical surface.

Further, in the above embodiment, it is preferable that the NA be notless than 0.85, and the objective lens having an NA of 0.85 is used asan example of an objective lens. However, even if the objective lens isdesigned such that a numerical aperture is slightly less than 0.85, theother conditions or specifications of the objective lens can be properlymodified to eliminate inconvenience caused by the NA slightly less than0.85. As a result, the objective lens may be used in an optical pickupdevice for an optical recording/reproducing apparatus based on astandard specification in which an NA thereof is 0.85.

For example, the number of the operation buttons and the optical pickupdevice, which are provided on the optical recording/reproducingapparatus according to the embodiment of the invention, is not limitedto the example shown in FIG. 15, and may be set optionally.

In the description of the embodiments, the blu-ray disk is used as anoptical recording medium, but the invention is not always limited tothis. The objective lens, the optical pickup device, and the opticalrecording/reproducing apparatus according to embodiment of the inventionmay be also applied to the case of using an optical recording medium fordifferent short wavelength light as the optical recording medium, forexample, so-called AOD (HD-DVD) disk or the like.

Furthermore, in the future, an optical recording medium having astandard in which a wavelength of the used light is further shortened toultraviolet region may be developed. However, even in this case, theinvention may be applied. In this case, as a lens material, it ispreferable to use a material having excellent transmittance to awavelength of the used light. For example, it is possible to usefluorite or quartz as a lens material of the objective lens according tothe invention.

1. An objective lens for converging light emitted from a light sourceonto an optical recording medium to record and reproduce information,the objective lens consisting of: a single lens having at least oneaspheric surface, wherein the following conditional expressions (1) to(3) are satisfied:N≦1.75  (1)0.5<f/f ₁<0.6  (2)0.8<d/(NA·De)<1.0  (3) where N denotes a refractive index of the singlelens, f denotes a focal length, in mm, of the single lens, f₁ denotes afocal length, in mm, of a light source side surface of the single lens,d denotes a thickness, in mm, of the single lens on an optical axis, NAdenotes a numerical aperture of the single lens on an optical recordingmedium side, and De denotes an effective aperture, in mm, of the lightsource side surface of the single lens.
 2. The objective lens accordingto claim 1, wherein the objective lens is formed in a biconvex shapenear the optical axis.
 3. The objective lens according to claim 1,wherein an optical recording medium side surface of the objective lensis formed in a convex shape near the optical axis, and is formed in aconcave shape at a predetermined position distant from the optical axis.4. The objective lens according to claim 1, wherein a mass of theobjective lens is equal to or less than 0.5 grams.
 5. The objective lensaccording to claim 1, wherein the following conditional expression (5)is satisfied:0.50<g/d<0.80  (5) where g denotes a distance, in mm, from a tangentialplane that is perpendicular to the optical axis and is tangent to avertex of the light source side surface of the single lens to a centerof gravity of the single lens.
 6. The objective lens according to claim1, wherein a numerical aperture of the single lens on the opticalrecording medium side is not less than 0.70 and not more than 0.98. 7.The objective lens according to claim 1, wherein a wavelength of thelight is not less than 400.0 nm and not more than 410.0 nm.
 8. Theobjective lens according to claim 7, wherein the numerical aperture ofthe single lens on the optical recording medium side is not less than0.85, and a thickness of a protection layer of the optical recordingmedium is not less than 0.075 mm and not more than 0.1 mm.
 9. Theobjective lens according to claim 7, wherein the numerical aperture ofthe single lens on the optical recording medium side is not less than0.85, an RMS of wavefront aberration is set to be minimized at aposition distant t1 mm from a surface of the optical recording mediumtoward the inside of the optical recording medium, and t1 is not lessthan 0.075 mm and not more than 0.1 mm.
 10. An optical pickup devicecomprising: the objective lens according to claim
 1. 11. An opticalrecording/reproducing apparatus comprising: the optical pickup deviceaccording to claim
 10. 12. An objective lens for converging lightemitted from a light source onto an optical recording medium to recordand reproduce information, the objective lens consisting of: a singlelens having at least one aspheric surface, wherein the followingconditional expressions (1), (2), and (4) are satisfied:N≦1.75  (1)0.5<f/f ₁<0.6  (2)0.3<d/Do<0.8  (4) where N denotes a refractive index of the single lens,f denotes a focal length, in mm, of the single lens, f₁ denotes a focallength, in mm, of a light source side surface of the single lens, ddenotes a thickness, in mm, of the single lens on an optical axis, andDo denotes an outer diameter, in mm, of the single lens.
 13. Theobjective lens according to claim 12, wherein the objective lens isformed in a biconvex shape near the optical axis.
 14. The objective lensaccording to claim 12, wherein an optical recording medium side surfaceof the objective lens is formed in a convex shape near the optical axis,and is formed in a concave shape at a predetermined position distantfrom the optical axis.
 15. The objective lens according to claim 12,wherein a mass of the objective lens is equal to or less than 0.5 grams.16. The objective lens according to claim 12, wherein the followingconditional expression (5) is satisfied:0.50<g/d<0.80  (5) where g denotes a distance, in mm, from a tangentialplane that is perpendicular to the optical axis and is tangent to avertex of the light source side surface of the single lens to a centerof gravity of the single lens.
 17. The objective lens according to claim12, wherein a numerical aperture of the single lens on the opticalrecording medium side is not less than 0.70 and not more than 0.98. 18.The objective lens according to claim 12, wherein a wavelength of thelight is not less than 400.0 nm and not more than 410.0 nm.
 19. Theobjective lens according to claim 18, wherein a numerical aperture ofthe single lens on the optical recording medium side is not less than0.85, and a thickness of a protection layer of the optical recordingmedium is not less than 0.075 mm and not more than 0.1 mm.
 20. Theobjective lens according to claim 18, wherein a numerical aperture ofthe single lens on the optical recording medium side is not less than0.85, an RMS of wavefront aberration is set to be minimized at aposition distant t1 mm from a surface of the optical recording mediumtoward the inside of the optical recording medium, and t1 is not lessthan 0.075 mm and not more than 0.1 mm.